This invention relates to aircraft systems and more particularly to an asymmetry brake for use in aircraft flight control actuation systems.
Aircraft commonly include movable flight control surfaces on their wings. These surfaces are known as flaps or slats, and can be selectively extended or retracted to modify the lift-producing characteristics of the wings. Extension and retraction of the flaps or slats is accomplished by a flight control actuation system mounted in each of the wings.
A typical flight control actuation system includes a series of actuators spaced along the span of each wing. The actuators are operably connected to move one or more flaps or slats. The actuators are connected to each other by drive shafts, to form a chain of actuators and shafts extending along the span of the wing. A power drive unit (PDU) connected to the inboard end of the chain provides motive power for driving the actuators to selectively extend or retract the flaps or slats.
Because the control surfaces such as the flaps or slats significantly alter the lift-producing characteristics of the wings, it is critical for safe operation of the aircraft that the actuation system also include safety features for detecting and reacting to problems such as jamming, shaft disconnect, or failure of one of the actuators in the aircraft flight control actuation system. Of particular concern are problems that cause the position of the flaps or slats on one wing to lose synchronization with the flaps and slats on the other wing of the aircraft. Such a condition is referred to as an asymmetry. To prevent asymmetry that cannot be controlled by the PDU alone, actuation systems for flaps and slats often include a device known as an asymmetry brake which engages to hold the chain of actuators and shafts in a known position when the asymmetry develops. For example, should a shaft connecting adjacent actuators break or disconnect, the PDU would not be able to control the position of flaps or slats outboard of the broken shaft. Without means to stabilize the position of the flaps or slats, such as an asymmetry brake at the outboard end of the chain of actuators and shafts, aerodynamic loads acting upon the flaps or slats could move them in a system where the actuators can be back driven by the air loads. The uncontrolled movement of the flaps or slats would create serious flight control problems for the aircraft.
In commercial aeronautical applications, asymmetry brakes are usually checked by the pilot during system power up. This requires that the asymmetry brake be engaged and disengaged remotely by an electrical controller, usually the actuation system controller located in the PDU. Thus, typical commercial flight control actuation systems employ a fail safe (spring actuated) electrical brake which can be remotely engaged by the pilot. However, electrical brakes by themselves are usually large and heavy with little brake torque capacity. Also, the location of the asymmetry brake is usually near the end of the wing where wing envelope and space are greatly reduced. Due to the reduced available wing envelope, fail safe electrical asymmetry brakes generally do not have the capacity to react the PDU motor stall torque for either normal pre-flight test conditions or other conditions. Because of this low capacity, a separate brake is usually located in the PDU to stop the PDU in case of drive line disconnect. This separate brake and the large fail safe asymmetry brakes add weight to the aircraft, decreasing aircraft operating efficiency.